JP2022550822A - Electrolyte for lithium secondary battery and lithium secondary battery containing the same - Google Patents

Abstract

Battery performance can be improved by combining highly reactive solvents and lithium salts, which can increase battery capacity, with highly stable solvents and lithium salts, which can increase battery life. An electrolyte for a lithium secondary battery and a lithium secondary battery containing the same are disclosed. The electrolyte for a lithium secondary battery includes a first solvent containing a heterocyclic compound containing at least one double bond and containing either one of an oxygen atom and a sulfur atom; and a fluorine-free ether. a second solvent containing any one or more of a compound, an ester compound, an amide compound and a carbonate compound; a third solvent containing a hydrofluoroether compound; a lithium salt; lanthanum nitrate; nitric acid Lithium and;

Description

This application claims the benefit of priority based on Korean Patent Application No. 10-2020-0059383 dated May 18, 2020, and all contents disclosed in the documents of that Korean Patent Application are incorporated herein by reference. incorporated as.

TECHNICAL FIELD The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery including the same, and more particularly, to a highly reactive solvent and a lithium salt capable of increasing battery capacity and increasing battery life. The present invention relates to an electrolytic solution for a lithium secondary battery and a lithium secondary battery containing the same, which can improve the performance of the battery by combining a highly stable solvent and a lithium salt.

With the increasing interest in energy storage technology, the fields of application have expanded to energy for mobile phones, tablets, laptops and camcorders, as well as electric vehicles (EV) and hybrid electric vehicles (HEV). Research and development on chemical elements are increasing. Electrochemical devices are a field that has received the most attention in this aspect, and among them, the development of secondary batteries such as rechargeable lithium-sulfur batteries has become a focus of interest. In order to improve the capacity density and specific energy in the development of a new battery, research and development into new electrode and battery designs have been conducted.

Among such electrochemical devices, a lithium-sulfur battery (Li-S battery) has a high energy density (theoretical capacity) and has been spotlighted as a next-generation secondary battery that can replace a lithium-ion battery. there is In such a lithium-sulfur battery, a reduction reaction of sulfur and an oxidation reaction of lithium metal occur during discharge. At this time, sulfur is converted from ring-structured _S8 to linear-structured lithium polysulfide (LiPS). ), such lithium-sulfur cells are characterized by a stepwise discharge voltage until the polysulfide is completely reduced to Li ₂ S.

However, the biggest obstacle in the commercialization of lithium-sulfur batteries is their lifespan. The causes of the deterioration of the life of such lithium-sulfur batteries include the side reactions of the electrolyte (accumulation of by-products due to the decomposition of the electrolyte), the instability of lithium metal (dendrite growth on the lithium negative electrode and the occurrence of short circuits). ) and cathode by-product deposition (lithium polysulfide elution from the cathode).

That is, in a battery in which a sulfur-based compound is used as a positive electrode active material and an alkali metal such as lithium is used as a negative electrode active material, elution and shuttle phenomenon of lithium polysulfide occurs during charging and discharging, and lithium polysulfide is transferred to the negative electrode. As a result, the capacity of the lithium-sulfur battery is reduced, and thus the lithium-sulfur battery has a serious problem of reduced life and reactivity. That is, since the polysulfide eluted from the positive electrode is highly soluble in the organic electrolyte, undesirable migration (PS shuttling) may occur toward the negative electrode through the electrolyte, resulting in irreversible deterioration of the positive electrode active material. Battery life is reduced due to capacity reduction due to loss and deposition of sulfur particles on the surface of lithium metal due to side reactions.

On the other hand, the behavior of the lithium-sulfur battery may vary greatly depending on the electrolyte. Catholye), and an electrolyte in which sulfur is hardly eluted in the form of lithium polysulfide is called sparingly soluble or solvating electrolyte (SSE). That is, in the industry, various researches on lithium-sulfur batteries in which sulfur, the positive electrode active material, is not eluted into the electrolyte In particular, research has been conducted on an electrolytic solution that can promote a Solid-to-Solid reaction with Li ₂ S, which is the final discharge product of sulfur. However, the reality is that we are still unable to produce results of this magnitude. Therefore, there is a need for a more fundamental method that can suppress the phenomenon that lithium polysulfide migrates to the negative electrode, shortening the life of the lithium-sulfur battery, and reducing the reactivity due to a large amount of lithium polysulfide.

Here, in the industry, as a part of preventing the side reaction problem of the electrolytic solution, the electrolytic solution containing an existing known ether-based solvent is replaced with a nitric acid-based solvent such as lanthanum nitrate (La(NO ₃ ) ₃ ). Attempts are being made to change the electrolyte to one containing compounds, but it is still difficult to find a fundamental solution.

That is, lanthanum nitrate has advantages such as improving the coulombic efficiency of lithium-sulfur batteries, and is often used as an electrolyte for lithium-sulfur batteries. In this connection, Non-Patent Document 1 discloses a lithium-sulfur battery using lanthanum nitrate (La(NO ₃ ) ₃ ) as an electrolyte additive for stabilizing the surface. Although electrolytes for lithium-sulfur batteries including 1,3-dioxolane (DOL), dimethoxyethane (DME), and even LiTFSI are disclosed, the problem of deterioration of lithium-sulfur battery life cannot be fundamentally improved. .

Therefore, it is necessary to develop a new electrolyte for lithium secondary batteries that can dramatically improve battery life performance by using lanthanum nitrate, which is effective in improving the coulombic efficiency of lithium-sulfur batteries. be done.

ACS APPLIED MATERIALS & INTERFACES 2016, 8, p. 7783-7789

It is therefore an object of the present invention to improve battery performance by combining highly reactive solvents and lithium salts that can increase battery capacity and highly stable solvents and lithium salts that can increase battery life. It is an object of the present invention to provide an electrolytic solution for a lithium secondary battery and a lithium secondary battery containing the same, which can improve the

To achieve the above objects, the present invention provides a first solvent containing a heterocyclic compound containing one or more double bonds and at the same time containing any one of an oxygen atom and a sulfur atom; a second solvent containing any one or more of an ether-based compound, an ester-based compound, an amide-based compound, and a carbonate-based compound; a third solvent including a hydrofluoroether-based compound; a lithium salt; and lanthanum nitrate. and lithium nitrate.

The present invention also provides a lithium secondary battery comprising: a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and the electrolyte for the lithium secondary battery.

According to the electrolyte for a lithium secondary battery and the lithium secondary battery including the same according to the present invention, a highly reactive solvent and a lithium salt that can increase the capacity of the battery and a stable electrolyte that can increase the life of the battery Battery performance can be improved by combining high-strength solvents and lithium salts. It has the advantage of improving battery performance such as cycle life by further including a compound.

4 is a graph showing coulombic efficiency, discharge capacity and life performance of a lithium secondary battery manufactured according to an embodiment of the present invention; 4 is a graph showing coulombic efficiency, discharge capacity and life performance of a lithium secondary battery manufactured according to an embodiment of the present invention; 4 is a graph showing coulombic efficiency, discharge capacity and life performance of a lithium secondary battery manufactured according to a comparative example;

The present invention will be described in detail below.

The electrolyte for a lithium secondary battery according to the present invention includes: A) a first solvent containing a heterocyclic compound containing at least one double bond and containing any one of an oxygen atom and a sulfur atom, and B) A second solvent containing at least one of fluorine-free ether-based compounds, ester-based compounds, amide-based compounds and carbonate-based compounds, C) a third solvent containing a hydrofluoroether-based compound, D) lithium salt , E) lanthanum nitrate and F) lithium nitrate.

As described above, the present applicant has developed a novel lithium secondary battery that can dramatically improve battery life performance by using lanthanum nitrate, which is effective in improving the coulombic efficiency of lithium-sulfur batteries. At the present time, when the development of electrolytes for batteries is required, we combine highly reactive solvents and lithium salts that can increase battery capacity and highly stable solvents and lithium salts that can increase battery life. In particular, lanthanum nitrate and hydrofluoroether compounds, which have advantages such as improving the coulombic efficiency of lithium-sulfur batteries, can be used as existing electrolyte components. We have developed an electrolyte for lithium secondary batteries that can improve battery performance such as cycle life by combining it with the compounds used.

That is, the electrolyte that can be applied to lithium secondary batteries such as lithium-sulfur batteries depends on the type of organic solvent and lithium salt (Li-Salt) contained in the electrolyte. lead to differences. Here, the Applicant has discovered that by combining highly reactive solvents and lithium salts that can increase battery capacity with highly stable solvents and lithium salts that can increase battery life. As a result of repeated research to find ways to improve performance,
i) The electrolytic solution contains a ``hydrofluoroether (HFE type) compound'' that has advantages such as improving the coulombic efficiency of lithium-sulfur batteries (exactly, it is used in the existing electrolytic solution components. partially replaced dimethoxyethane (DME)),
ii) Similarly, the electrolytic solution contains "lanthanum nitrate (La( _NO3 ) ₃ )", which has advantages such as improving the coulombic efficiency of lithium-sulfur batteries. part of the _LiNO3 used in the liquid component),
iii) 1,3-dioxolane (DOL), which has been used as a component of the existing electrolyte solution, has been replaced with "a heterocyclic compound containing one or more double bonds and at the same time containing either one of an oxygen atom and a sulfur atom. Change to "solvent containing (first solvent)",
iv) Similarly, by replacing LiTFSI used in existing electrolyte components with 'other lithium salts such as LiFSI', the inventors have come up with the present invention in which reactivity and life are improved.

Hereinafter, each of A) first solvent, B) second solvent, C) third solvent, D) lithium salt, E) lanthanum nitrate and F) lithium nitrate contained in the electrolyte solution for a lithium secondary battery of the present invention A specific description will be given.

A) First solvent The first solvent contains a heterocyclic compound containing one or more double bonds and at the same time containing any one of an oxygen atom and a sulfur atom, wherein the heteroatom (oxygen Due to the delocalization of lone pair electrons (atoms or sulfur atoms), it is difficult to dissolve salts. Formation of lithium dendrites can be suppressed by forming a polymer protective film (solid electrolyte interface, SEI layer) on the surface of the lithium-based metal (negative electrode) by (ring opening reaction). It is possible to improve the life characteristics of the lithium-sulfur battery by reducing the decomposition of the electrolyte and the resulting side reactions.

That is, the heterocyclic compound of the present invention must contain at least one double bond in order to form a polymer protective film on the surface of the lithium-based metal, and must be polar in the electrolyte. A heteroatom (oxygen atom or sulfur atom) must also be included so as to exhibit effects such as enhancing affinity with other solvents.

The heterocyclic compound may be a 3- to 15-membered, preferably 3- to 7-membered, more preferably 5- to 6-membered heterocyclic compound. In addition, such heterocyclic compounds include an alkyl group having 1 to 4 carbon atoms, a cyclic alkyl group having 3 to 8 carbon atoms, an aryl group having 6 to 10 carbon atoms, a halogen group, a nitro group (-NO ₂ ), an amine It is a heterocyclic compound substituted or unsubstituted by one or more selected from the group consisting of a group (-NH ₂ ) and a sulfonyl group (-SO ₂ ). In addition, the heterocyclic compound is a polycyclic compound of a heterocyclic compound and at least one of a cyclic alkyl group having 3 to 8 carbon atoms and an aryl group having 6 to 10 carbon atoms.

When the heterocyclic compound is substituted with an alkyl group having 1 to 4 carbon atoms, the radical is stabilized and side reactions between electrolytes can be suppressed, which is preferable. In addition, when substituted with a halogen group or a nitro group, a functional protective film can be formed on the surface of the lithium-based metal, and at this time, the formed functional protective film is in a compact form. It is stable as a protective film of , enables uniform deposition of lithium-based metal, and inhibits side reaction between polysulfide and lithium-based metal.

Specific examples of the heterocyclic compound include furan, 2-methylfuran, 3-methylfuran, 2-ethylfuran, 2-propylfuran (2-propylfuran), 2-butylfuran, 2,3-dimethylfuran (2,3-dimethylfuran), 2,4-dimethylfuran (2,4-dimethylfuran), 2,5-dimethylfuran (2,5-dimethylfuran), pyran, 2-methylpyran, 3-methylpyran, 4-methylpyran, benzofuran, 2-(2- 2-(2-Nitrovinyl)furan, thiophene, 2-methylthiophene, 2-ethylthiophene, 2-propylthiophene, 2 - 2-butylthiophene, 2,3-dimethylthiophene, 2,4-dimethylthiophene and 2,5-dimethylthiophene One or more selected from the group consisting of, but not limited to, 2-methylfuran is preferably included in the first solvent.

B) Second solvent The second solvent contains one or more of fluorine-free ether-based compounds, ester-based compounds, amide-based compounds and carbonate-based compounds, and dissolves the lithium salt. In the case of the carbonate-based compound, it not only makes the electrolyte have lithium ion conductivity, but also dissolves sulfur, which is a positive electrode active material, to facilitate an electrochemical reaction with lithium. , a linear carbonate-based compound or a cyclic carbonate-based compound. On the other hand, the compounds containing no fluorine in the second solvent are limited to ether-based compounds, and the remaining ester-based compounds, amide-based compounds and carbonate-based compounds may contain fluorine.

Specific examples of the fluorine-free ether compounds include dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane, diethylene glycol dimethyl ether, Diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, polyethylene glycol dimethyl ether, polyethylene One or more selected from the group consisting of glycol diethyl ether and polyethylene glycol methyl ethyl ether can be used, but the second solvent is not limited thereto, and dimethoxyethane is preferably included as the second solvent.

The ester compounds include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone and One or more selected from the group consisting of ε-caprolactone can be mentioned, but not limited thereto. Also, the amide-based compound is a common amide-based compound used in the industry.

In addition, the linear carbonate-based compounds include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethylmethyl carbonate (EMC), methylpropyl One or more selected from the group consisting of carbonate (MPC) and ethylpropyl carbonate (EPC) can be used, but not limited thereto.

The cyclic carbonate compounds include ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2 , 3-pentylene carbonate, vinylene carbonate, vinylethylene carbonate, and halides thereof (fluoroethylene carbonate, FEC, etc.). not to be

C) Third Solvent The third solvent contains a hydrofluoroether (HFE type) compound and has the effect of inhibiting the dissolution and solvent decomposition of polysulfides, thereby improving the coulombic efficiency of the battery. E.) and ultimately serve to improve battery life.

Specific examples of the hydrofluoroether compounds include bis(fluoromethyl) ether, 2-fluoromethyl ether, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, Bis(2,2,2-trifluoroethyl) ether, propyl 1,1,2,2-tetrafluoroethyl ether, isopropyl 1,1,2,2-tetrafluoroethyl ether, 1,1,2,2- Tetrafluoroethyl isobutyl ether, 1,1,2,3,3,3-hexafluoropropyl ethyl ether, 1H,1H,2'H,3H-decafluorodipropyl ether and 1H,1H,2'H-perfluoro One or more selected from the group consisting of dipropyl ether can be exemplified, and among these, bis(2,2,2-trifluoroethyl)ether (BTFE) is preferably included as an essential component.

Regarding the contents of the first solvent, the second solvent and the third solvent as described above, the content of the first solvent is 5 to 5 based on the total contents of the first solvent, the second solvent and the third solvent. 50 vol%, preferably 10 to 30 vol%, the content of the second solvent is 45 to 90 vol%, preferably 60 to 80 vol%, the content of the third solvent is 1 to 10 vol%, preferably 5 to 10% by volume.

On the other hand, if the amount of the third solvent exceeds 10% by volume, the resistance may increase, which may act as a factor to reduce the discharge capacity of the battery, and the third solvent may partially replace the second solvent. Therefore, the mixing ratio of the second solvent and the third solvent is 7 to 79:1, preferably 7 to 15:1 in terms of volume ratio.

If the amount of the first solvent is less than 5% by volume based on the total organic solvent, the ability to reduce the amount of polysulfide elution is reduced, and the increase in the resistance of the electrolyte cannot be suppressed. A problem may arise that the protective film is not completely formed on the metal surface. In addition, if the amount of the first solvent exceeds 50% by volume based on the total amount of the organic solvent, the surface resistance of the electrolyte and the lithium-based metal increases, which may lead to a decrease in the discharge capacity of the battery.

In addition, if the second solvent is less than 45% by volume based on the total organic solvent, the lithium salt cannot be sufficiently dissolved, resulting in a decrease in lithium ion conductivity and dissolution of sulfur, which is an active material. If the concentration exceeds 90% by volume, sulfur, which is the active material, is eluted too much, and the shuttle phenomenon between the lithium polysulfide and the lithium negative electrode becomes severe. can cause a problem of decreasing

In addition, if the third solvent is included in less than 1% by volume of the total organic solvent, the ability to suppress the elution of lithium polysulfide may be insufficient. A large overvoltage may occur due to a decrease in mass transfer resistance and ionic conductivity due to the solvent.

D) Lithium salt The lithium salt is an electrolyte salt used to increase ionic conductivity, and can be used without limitation as long as it is commonly used in the art. Specific examples _of said lithium salts include LiCl, LiBr, LiI, _{LiClO4 , LiBF4} , _LiB10Cl10 , _LiPF6 , _{LiCF3SO3 , LiCF3CO2 , LiC4BO8} , _LiAsF6 , _LiSbF6 , _LiAlCl4 , _{CH3SO3Li , CF3SO3Li} , ( _{CF3SO2 ) 2NLi ,} ( _{C2F5SO2 )} 2NLi _, ( _SO2F ) _2NLi , _{( CF3SO2 ) 3} CLi, lithium chloroborane, lithium lower aliphatic carboxylate having 4 or less carbon atoms, lithium 4-phenylborate and one or more selected from the group consisting of lithium imide, and LiFSI ( ( _SO2F ) _2NLi ) is preferably used as an essential component. At the same time, as an embodiment of the present invention, the electrolyte may be in a form that does not contain LiTFSI ((CF ₃ SO ₂ ) ₂ NLi).

The concentration of the lithium salt can be determined in consideration of ionic conductivity, and is, for example, 0.1 to 2M, preferably 0.5 to 1M, more preferably 0.5 to 0.75M. When the concentration of the lithium salt is less than the above range, it is difficult to ensure ionic conductivity suitable for driving the battery. The decomposition reaction of the lithium salt itself may increase, resulting in deterioration of battery performance.

E) Lanthanum Nitrate The lanthanum nitrate (La(NO ₃ ) ₃ ) is a component used to improve the coulombic efficiency (C.E.) of the battery and ultimately the life of the battery. As mentioned above, it has been used as an electrolyte component in the past, but the relevant cases included 1,3-dioxolane (DOL), dimethoxyethane (DME) and LiTFSI in addition to lanthanum nitrate. However, it was not possible to fundamentally solve the problem of deterioration of battery life (that is, it is difficult for lanthanum nitrate to exhibit its effect in an electrolyte containing a mixture of DOL/DME/LiTFSI).

However, although the present invention uses lanthanum nitrate (La(NO ₃ ) ₃ ), it replaces 1,3-dioxolane (DOL), which was used in the existing electrolyte components, with the idea that it contains one or more double bonds. At the same time, the solvent containing a heterocyclic compound containing either one of oxygen atoms and sulfur atoms (first solvent)" was changed, and part of the content occupied by dimethoxyethane (DME) was changed to "hydrofluoro Ether type (HFE Type)”, and LiTFSI used in existing electrolyte components was changed to “other lithium salts such as LiFSI” to improve reactivity and life. did.

Lanthanum nitrate as described above may be included in an amount of 1 to 10 wt%, preferably 2 to 7 wt%, more preferably 4 to 6 wt%, based on the total weight of the electrolyte for a lithium secondary battery. If the lanthanum nitrate content is less than 1% by weight with respect to the total weight of the electrolyte, the coulombic efficiency of the battery will be marginally improved, and the life of the battery will be marginally improved. It is possible that more than 10% by weight may not show any further benefit from the use of lanthanum nitrate.

F) Lithium Nitrate Further, the electrolyte for a lithium secondary battery according to the present invention basically contains lithium nitrate (LiNO ₃ ). However, if necessary, potassium nitrate (KNO ₃ ), cesium nitrate (CsNO ₃ ), magnesium nitrate (Mg(NO ₃ ) ₂ ), barium nitrate (Ba(NO ₃ ) ₂ ), lithium nitrite (LiNO ₂ ), At least one selected from the group consisting of potassium nitrite (KNO ₂ ) and cesium nitrite (CsNO ₂ ) may be further included.

The lithium nitrate may be included in an amount of 0.5 to 10 wt%, preferably 1 to 6 wt%, more preferably 2 to 4 wt%, based on the total weight of the electrolyte for a lithium secondary battery. If the content of lithium nitrate is less than 0.5 wt% based on the total weight of the electrolyte for a lithium secondary battery, the coulombic efficiency may decrease sharply, and if it exceeds 10 wt%, the viscosity of the electrolyte may decrease. becomes high and difficult to drive. Meanwhile, the total content of lithium nitrate and lanthanum nitrate is preferably about 2 to 10% by weight based on the total weight of the electrolyte for a lithium secondary battery, and the content ratio of lithium nitrate and lanthanum nitrate. is in a weight ratio of 1 to 5:5 to 1, but is not limited thereto.

Next, a lithium secondary battery according to the present invention will be described. The lithium secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte for the lithium secondary battery. As described above, the electrolytic solution for a lithium secondary battery contains A) a first solvent, B) a second solvent, C) a third solvent, D) a lithium salt, E) lanthanum nitrate and F) lithium nitrate. The above-mentioned contents apply mutatis mutandis to the specific contents. In addition, the lithium secondary battery may be any lithium secondary battery commonly used in the industry, among which the lithium-sulfur battery is most preferred.

Hereinafter, the positive electrode, negative electrode and separator in the lithium secondary battery according to the present invention will be described in more detail.

As described above, the positive electrode included in the lithium secondary battery of the present invention includes a positive electrode active material, a binder, a conductive material, and the like. The positive active material may be applied to a general lithium secondary battery, and may include, for example, elemental sulfur ( _S8 ), a sulfur-based compound, or a mixture thereof. Specifically, the sulfur-based compound is Li ₂ S _n (n≧1), an organic sulfur compound or a carbon-sulfur complex ((C ₂ S _x ) _n : x=2.5 to 50, n≧2), etc. be. In addition, the cathode active material may include a sulfur-carbon composite, and since the sulfur material alone has no electrical conductivity, it may be used in combination with a conductive material. The carbon material (or carbon source) constituting the sulfur-carbon composite has a porous structure or a high specific surface area, and any material commonly used in the industry may be used. For example, the porous carbon material includes graphite; graphene; carbon black such as Denka black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; single-walled carbon nanotubes ( Carbon nanotubes (CNT), such as SWCNT), multi-walled carbon nanotubes (MWCNT); carbon fibers, such as graphite nanofibers (GNF), carbon nanofibers (CNF), activated carbon fibers (ACF); and activated carbon. It may be selected from one or more types, but is not limited thereto, as long as the shape is spherical, rod-shaped, needle-shaped, plate-shaped, tube-shaped or bulk-shaped and is commonly used in lithium secondary batteries. Can be used without restrictions.

Also, the carbon material has pores, and the porosity of the pores is 40 to 90%, preferably 60 to 80%. Since the transmission is not performed normally, it may act as a resistance component and cause problems. In addition, the pore size of the carbon material is 10 nm to 5 μm, preferably 50 nm to 5 μm, and if the pore size is less than 10 nm, lithium ions cannot permeate. However, if the thickness exceeds 5 μm, battery short circuit and safety problems may occur due to contact between electrodes.

The binder is a component that assists the binding of the positive electrode active material and the conductive material and the binding to the current collector. ), polyvinyl acetate, polyvinyl alcohol, polyvinyl ether, polyethylene, polyethylene oxide, alkylated polyethylene oxide, polypropylene, polymethyl (meth) acrylate, polyethyl (meth) acrylate, polytetrafluoroethylene (PTFE), polyvinyl chloride, polyacrylonitrile, Polyvinylpyridine, polyvinylpyrrolidone, styrene-butadiene rubber, acrylonitrile-butadiene rubber, ethylene-propylene-diene monomer (EPDM) rubber, sulfonated EPDM rubber, styrene-butylene rubber, fluororubber, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose , regenerated cellulose, and mixtures thereof, but are not necessarily limited thereto.

The binder is generally added in an amount of 1 to 50 parts by weight, preferably 3 to 15 parts by weight, based on 100 parts by weight of the total weight of the positive electrode. If the content of the binder is less than 1 part by weight, the adhesion between the positive electrode active material and the current collector may be insufficient. As the active material content decreases, the battery capacity decreases.

The conductive material contained in the positive electrode is not particularly limited as long as it has excellent electrical conductivity without causing side reactions in the internal environment of the lithium secondary battery and without causing chemical changes in the battery. For example, graphite or conductive carbon can be used, for example, graphite such as natural graphite and artificial graphite; carbon black, acetylene black, ketjen black, denka black, thermal black, channel black, furnace black, lamp carbon black such as black; carbon-based substances whose crystal structure is graphene or graphite; conductive fibers such as carbon fiber and metal fiber; carbon fluoride; metal powder such as aluminum powder and nickel powder; zinc oxide, potassium titanate, etc. conductive whiskers; conductive oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives; do not have.

The conductive material is generally added in an amount of 0.5 to 50 parts by weight, preferably 1 to 30 parts by weight, based on 100 parts by weight of the total weight of the positive electrode. If the content of the conductive material is less than 0.5 parts by weight, the effect of improving the electrical conductivity may not be expected, or the electrochemical characteristics of the battery may be deteriorated. If the amount exceeds the weight part, the amount of the positive electrode active material is relatively small, resulting in a decrease in capacity and energy density. The method of including the conductive material in the positive electrode is not particularly limited, and a conventional method known in the art such as coating the positive electrode active material can be used. Also, if necessary, the addition of the conductive material as described above can be replaced by adding a conductive second coating layer to the positive electrode active material.

In addition, a filler may be selectively added to the positive electrode of the present invention as a component that suppresses the expansion thereof. Such a filler is not particularly limited as long as it can suppress the expansion of the electrode without causing a chemical change in the battery. Examples include olefinic polymers such as polyethylene and polypropylene; glass fiber; A fibrous material such as carbon fiber; and the like can be used.

The positive electrode is manufactured by dispersing and mixing a positive electrode active material, a binder, a conductive material, etc. in a dispersion medium (solvent) to prepare a slurry, applying the slurry on a positive electrode current collector, drying and rolling the slurry. be able to. The dispersion medium includes N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol, isopropanol, water and these Mixtures can be used, but are not necessarily limited to this.

The positive electrode current collector includes platinum (Pt), gold (Au), palladium (Pd), iridium (Ir), silver (Ag), ruthenium (Ru), nickel (Ni), stainless steel (STS), aluminum ( Al), molybdenum (Mo), chromium (Cr), carbon (C), titanium (Ti), tungsten (W), ITO (In doped SnO ₂ ), FTO (F doped SnO ₂ ), and alloys thereof; Aluminum (Al) or stainless steel surface treated with carbon (C), nickel (Ni), titanium (Ti) or silver (Ag) may be used, but is not necessarily limited thereto. do not have. The positive electrode current collector may be in the form of foil, film, sheet, punched material, porous material, foam, or the like.

The negative electrode is a lithium-based metal, and may further include a current collector on one side of the lithium-based metal. A negative current collector may be used as the current collector. The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. It can be selected from the group consisting of these alloys and combinations thereof. The stainless steel can be surface treated with carbon, nickel, titanium or silver; A flexible polymer or a conductive polymer can also be used. In general, a thin copper plate is applied for the negative electrode current collector.

In addition, various forms such as films, sheets, foils, nets, porous bodies, foams, non-woven fabrics, etc. with or without fine irregularities formed on the surface can be used. Also, the negative electrode current collector has a thickness ranging from 3 to 500 μm. If the thickness of the negative electrode current collector is less than 3 μm, the current collection effect is reduced. be.

The lithium-based metal is lithium or a lithium alloy. At this time, the lithium alloy includes an element that can be alloyed with lithium, specifically lithium and Si, Sn, C, Pt, Ir, Ni, Cu, Ti, Na, K, Rb, Cs, Fr, Be, Mg. , Ca, Sr, Sb, Pb, In, Zn, Ba, Ra, Ge and Al.

The lithium-based metal is in the form of a sheet or foil, and in some cases, lithium or a lithium alloy is deposited or coated on a current collector by a dry process, or a particulate metal or alloy is deposited by a wet process. or in coated form.

A common separator may be interposed between the positive electrode and the negative electrode. The separation membrane is a physical separation membrane having a function of physically separating the electrodes, and can be used without particular limitation as long as it is used in a normal separation membrane, especially ion migration of the electrolyte. It is preferable to use a material that has a low resistance to the electrolyte and is excellent in the ability to absorb moisture from the electrolytic solution.

In addition, the separation membrane separates or insulates the positive electrode and the negative electrode from each other and allows lithium ions to be transported between the positive electrode and the negative electrode. Such separation membranes are made of porous, non-conducting or insulating materials. The separator is an independent member such as a film, or a coating layer added to the positive electrode and/or the negative electrode.

Examples of polyolefin-based porous membranes that can be used in the separation membrane include polyethylenes such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, ultra-high molecular weight polyethylene, and polyolefin-based membranes such as polypropylene, polybutylene, and polypentene. Membranes formed by using a polymer alone or a mixture of these polymers can be mentioned. Examples of nonwoven fabrics that can be used in the separation membrane include polyphenyleneoxide, polyimide, polyamide, polycarbonate, polyethyleneterephthalate, polyethylenenaphthalate, polybutylene. Polymers such as terephthalate, polyphenylene sulfide, polyacetal, polyethersulfone, polyetheretherketone, polyester, etc. are formed alone or as a mixture thereof. Non-woven fabrics are possible, and such non-woven fabrics include spunbond or meltblown forms made up of long fibers in the form of fibers that form a porous web.

Although the thickness of the separator is not particularly limited, it is preferably in the range of 1 to 100 μm, more preferably in the range of 5 to 50 μm. If the thickness of the separator is less than 1 μm, the mechanical properties cannot be maintained. Although the pore size and porosity of the separator are not particularly limited, the pore size is preferably 0.1 to 50 μm and the porosity is 10 to 95%. When the pore size of the separation membrane is less than 0.1 μm or the porosity is less than 10%, the separation membrane acts as a resistance layer, and the pore size exceeds 50 μm or the porosity is less than 95%. If it exceeds, the mechanical properties cannot be maintained.

The lithium secondary battery of the present invention comprising the positive electrode, the negative electrode, the separation membrane and the electrolyte as described above is obtained by placing the positive electrode facing the negative electrode and interposing the separation membrane therebetween, followed by can be manufactured through a process of injecting

On the other hand, the lithium secondary battery according to the present invention can be applied not only as a battery cell used as a power source for small devices, but also as a unit battery of a battery module as a power source for medium and large devices. In this aspect, the present invention also provides a battery module including two or more lithium secondary batteries electrically connected (in series or parallel). Of course, the number of lithium secondary batteries included in the battery module can be variously adjusted in consideration of the usage and capacity of the battery module. Further, the present invention provides a battery pack in which the battery modules are electrically connected according to the ordinary skill in the art. The battery modules and battery packs are used in power tools; electric vehicles (EV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (PHEV). ); electric trucks; electric commercial vehicles; or systems for power storage.

Hereinafter, preferred embodiments are presented to facilitate understanding of the present invention, but the following embodiments are merely illustrative of the present invention, and various changes and modifications can be made within the scope and technical spirit of the present invention. It is obvious to a person skilled in the art that such changes and modifications are possible and should be covered by the appended claims.

[Example 1] Production of lithium secondary battery Electrolyte production First, 2-methylfuran (first solvent), dimethoxyethane (second solvent), third solvent) at a volume ratio (v/v) of 20:79:1. 3% by weight of lithium nitrate (LiNO ₃ ) and 5% by weight of lanthanum nitrate (La(NO ₃ ) ₃ ) were added to prepare an electrolyte for a lithium secondary battery. Here, the lanthanum nitrate (La(NO ₃ ) ₃ ) was prepared by vacuum-drying La(NO ₃ ) _{3.6H 2} O from Aldrich at 180° C. for 18 hours to remove H ₂ O. is.

Positive electrode manufacturing Positive electrode active material sulfur-carbon (CNT) composite (S/C 70:30 weight ratio) 90 parts by weight, conductive material Denka black 5 parts by weight, binder styrene-butadiene rubber/carboxymethyl cellulose (SBR/CMC 7) 3) After preparing a positive electrode slurry composition by mixing 5 parts by weight, the prepared slurry composition was coated on a current collector (Al foil) and dried at 50° C. for 12 hours. A positive electrode was manufactured by pressing with a roll press machine (at this time, the loading amount was 3.5 mAh/cm ² and the porosity of the electrode was 65%).

Manufacture of a lithium secondary battery (lithium-sulfur battery) The positive electrode and the lithium metal negative electrode having a thickness of 150 μm were positioned to face each other, and a polyethylene (PE) separator was interposed therebetween. A coin cell type lithium-sulfur battery was manufactured by injecting an electrolyte. Meanwhile, in manufacturing the battery, the positive electrode was used by punching out a circular electrode of 14 phi, the polyethylene separator was punched out of 19 phi, and the lithium metal was punched out of 16 phi.

[Examples 2 to 6, Comparative Examples 1 to 4] Production of Lithium Secondary Battery The procedure of Examples 2 to 4 was carried out in the same manner as in Example 1, except that the composition of the electrolyte solution was changed as shown in Table 1 below. 6 and Comparative Examples 1 to 4 were manufactured.

[Experimental Example 1] Evaluation of coulombic efficiency of lithium secondary batteries The lithium secondary batteries (specifically, lithium-sulfur batteries) manufactured in Examples 1 to 6 were charged at 0.1C for 3 cycles and After discharging, the battery was charged at 0.2C and discharged at 0.3C to evaluate the coulombic efficiency of the battery. At this time, the voltage range used was 1.8 to 2.5V (that is, discharge to 1.8V and charge to 2.5V), and the evaluation temperature was 25°C.

1 and 2 are graphs showing the coulombic efficiency, discharge capacity and life performance of a lithium secondary battery manufactured according to an embodiment of the present invention, in which a part of the dimethoxyethane content in the electrolyte is replaced with BTFE. Alternatively, the lithium-sulfur batteries of Examples 1 to 6, in which both lanthanum nitrate (La(NO ₃ ) ₃ ) and LiFSI were applied, all exhibited excellent coulombic efficiency as shown in FIGS. The higher the BTFE content, the higher the coulombic efficiency. Through this, it was confirmed that the coulombic efficiency, which affects the life performance of the lithium-sulfur battery, increases as the content of BTFE increases.

[Experimental Example 2] Evaluation of life characteristics and discharge capacity of lithium secondary batteries After the battery was charged and discharged for a period of time, it was then charged at 0.2C and discharged at 0.3C to evaluate the life characteristics and discharge capacity of the battery. At this time, the voltage range used was 1.8 to 2.5V (that is, discharge to 1.8V and charge to 2.5V), and the evaluation temperature was 25°C.

FIG. 3 is a graph showing the coulombic efficiency, discharge capacity and life performance of the lithium secondary battery manufactured according to Comparative Example. The lithium-sulfur battery of Example 1, in which the part was replaced with BTFE and both lanthanum nitrate (La(NO ₃ ) ₃ ) and LiFSI were applied, was the lithium-sulfur battery of Comparative Examples 1 to 4, which did not contain BTFE in the electrolyte. - Excellent coulombic efficiency while maintaining similar discharge capacity as compared to sulfur battery (also excellent in life characteristics compared to Comparative Examples 1 and 2). Through this, it was found that the use of BTFE increases the coulomb efficiency and improves the life performance of the lithium-sulfur battery.

Next, looking at the life characteristics and discharge capacity of Examples 1 to 6 with reference to FIGS. can be confirmed. However, in the case of Example 3 in which BTFE was used at 10% by volume among Examples 1 to 3 in which 0.75 M of LiFSI was used, the discharge capacity was slightly lower than in Examples 1 and 2, but LiFSI was used at 0.5M and BTFE was used at 10% by volume. Therefore, it can be seen that it is preferable to use LiFSI contained in the electrolytic solution of the present invention at a concentration as close to 0.5M as possible.

Summarizing the above, in the electrolytic solution of the present invention, the third solvent such as BTFE replaces a part of the second solvent such as dimethoxyethane, and the coulombic efficiency of the battery increases as the content of the third solvent increases. And life performance can be improved. However, as the content of the third solvent increases, the resistance increases and the discharge capacity may decrease. Therefore, the concentration of the lithium salt (LiFSI) and the content of the third solvent must be set within the scope of the present invention. is required.

Claims (15)

a first solvent containing a heterocyclic compound containing at least one double bond and at the same time containing any one of an oxygen atom and a sulfur atom;
a second solvent containing at least one of fluorine-free ether-based compounds, ester-based compounds, amide-based compounds, and carbonate-based compounds;
a third solvent containing a hydrofluoroether compound;
a lithium salt;
lanthanum nitrate;
An electrolyte for a lithium secondary battery, comprising lithium nitrate; Based on the total content of the first solvent, the second solvent and the third solvent, the content of the first solvent is 5 to 50% by volume, the content of the second solvent is 45 to 90% by volume, and the first The electrolyte for a lithium secondary battery according to claim 1, wherein the content of the three solvents is 1 to 10% by volume. 2. The electrolyte solution for a lithium secondary battery according to claim 1, wherein the mixing ratio of the second solvent and the third solvent is 7 to 79:1 in terms of volume ratio. The hydrofluoroether compounds include bis(fluoromethyl) ether, 2-fluoromethyl ether, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, bis(2,2 ,2-trifluoroethyl) ether, propyl 1,1,2,2-tetrafluoroethyl ether, isopropyl 1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl isobutyl ether , 1,1,2,3,3,3-hexafluoropropylethyl ether, 1H,1H,2′H,3H-decafluorodipropyl ether and 1H,1H,2′H-perfluorodipropyl ether 4. The electrolytic solution for a lithium secondary battery according to any one of claims 1 to 3, wherein the electrolytic solution is one or more selected from the group. The heterocyclic compound includes furan, 2-methylfuran, 3-methylfuran, 2-ethylfuran, 2-propylfuran, 2-butylfuran, 2,3-dimethylfuran, 2,4-dimethylfuran, 2,5 -dimethylfuran, pyran, 2-methylpyran, 3-methylpyran, 4-methylpyran, benzofuran, 2-(2-nitrovinyl)furan, thiophene, 2-methylthiophene, 2-ethylthiophene, 2-propylthiophene, 2-butylthiophene , 2,3-dimethylthiophene, 2,4-dimethylthiophene and 2,5-dimethylthiophene. The lithium salts include LiCl, LiBr, LiI, _LiClO4 , _LiBF4 , _LiB10Cl10 , _LiPF6 , _{LiCF3SO3 , LiCF3CO2 , LiC4BO8} , _{LiAsF6 , LiSbF6} , _{LiAlCl4 ,} CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, (C ₂ F ₅ SO ₂ ) ₂ NLi, LiFSI ((SO ₂ F) ₂ Nli), (CF ₃ SO ₂ ) ₃ CLi, lithium chloroborane, 4 or less carbon atoms Lithium lower aliphatic carboxylate, lithium 4-phenylborate and lithium imide. 7. The electrolyte for a lithium secondary battery according to claim 6, wherein the lithium salt contains LiFSI. The electrolyte for a lithium secondary battery according to claim 1, wherein the lithium salt has a concentration of 0.1 to 2M. The lithium secondary battery electrolyte further comprises one or more selected from the group consisting of potassium nitrate, cesium nitrate, magnesium nitrate, barium nitrate, lithium nitrite, potassium nitrite and cesium nitrite. The electrolytic solution for a lithium secondary battery according to claim 1. 2. The electrolyte for lithium secondary batteries according to claim 1, wherein the total content of said lithium nitrate and lanthanum nitrate is 2 to 10% by weight based on the total weight of said electrolyte for lithium secondary batteries. liquid. The fluorine-free ether compounds include dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, and diethylene glycol. Methyl ethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, polyethylene glycol dimethyl ether, polyethylene glycol diethyl ether and polyethylene 2. The electrolyte for a lithium secondary battery according to claim 1, wherein the electrolyte is at least one selected from the group consisting of glycol methyl ethyl ether. The electrolyte for lithium secondary batteries includes 2-methylfuran as a first solvent, dimethoxyethane as a second solvent, bis(2,2,2-trifluoroethyl) ether as a third solvent, LiFSI, nitric acid 2. The electrolyte for a lithium secondary battery according to claim 1, comprising lanthanum and lithium nitrate. The electrolyte for a lithium secondary battery according to claim 1, wherein the electrolyte is for a lithium-sulfur battery. A lithium secondary battery comprising: a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and the electrolyte for a lithium secondary battery according to claim 1 . The lithium secondary battery according to claim 14, wherein the lithium secondary battery is a lithium-sulfur battery.

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